The invention concerns an oil pump for motor vehicle transmissions.
Automatic or continuously variable transmissions of motor vehicles require an oil pump for the supply of the lubrication device and of the control and actuation devices, which is often designed as a displacement pump and is driven at the rotational speed of the prime mover of the motor vehicle.
The oil pump is still today predominantly designed as a so-called constant pump whose flow rate increases in proportion to its driving rotational speed. Here the pump is generally designed based on the idling speed of the engine. The delivered flow rate already must meet the requirements of the automatic transmission to be supplied. However, at high rotational speeds of the engine, such a constant pump, delivers many times the amount required. Thereby such constant pumps consume too much power, cavitate and produce an excessive level of noise. In addition, the duct cross sections have to be extensively overdimensioned.
In automatic transmissions, so-called variable displacement pumps, have already been used which have a flow rate better adaptable to supply needs but, which in the lower to middle rotational speed of the engine, which are relevant for consumption, have a poorer degree of efficiency than a constant pump. Moreover, they deliver too little oil at low rotational speeds of the engine.
There have already been proposed suction-controlled constant pumps for automatic transmissions of passenger motor vehicles (EP 0 682 756), to overcome the disadvantage, specifically in the form of a suction-throttled radial piston pump which, starting from a certain rotational speed, delivers an almost constant flow rate which also requires almost constant powerconsumption. A suction-throttled radial piston pump has the disadvantage of an elevated development of noise.
Finally, pumps purely electrically operated have also been installed in automatic transmissions for passenger vehicles, but they have the disadvantage of heavily loading the electric on-board network of the passenger car when they exclusively must ensure the total oil supply of the automatic transmission.
Mechanically driven 2-pump systems (tandem pumps) are also known in which one of the two pumps is switched to pressureless delivery at high rotational speeds (Cadillac plant). But, they also deliver too little oil in the lower rotational speed range and cannot be properly controlled. The insufficient delivery amount at low rotational speeds can be explained by the fact that the volumetric degree of efficiency is very poor and, therefore, the use of two slowly rotating pumps is not logical.
The problem on which this invention is based is to provide an oil pump for motor vehicle transmissions which has a low level of noise and, which at every rotational speed range, makes the needed flow rate.
On the basis of an oil pump of the kind mentioned in detail above, this problem is solved.
The oil pump is provided with a lesser displacement volume in comparison with the purely mechanically operated oil pumps; the use of an electric driving part makes it possible, at low engine rotational speeds, to bring the oil pump to higher rotational speeds and thus to deliver more oil at these low rotational speeds. In the low rotational speed range, i.e. in the range of up to 1500 rpm, the pump is exclusively driven by the electric driving part. A provided freewheel makes it possible for the rotational speed of the oil pump to be higher than the rotational speed of the internal combustion engine. With a rotational speed range, beginning at approximately 1500 rpm, the internal combustion engine of the motor vehicle rotates quicker than the electromotor that constitutes the electric driving part. The internal combustion engine now locks to the freewheel and assumes the actuation of the oil pump. The electric driving part is simultaneously disconnected.
In one other embodiment of this invention, the electric driving part is an independent electric pump which, together with the mechanically driven pump, is connected with a valve device which supplies the automatic transmission with the required oil current. In this case as in the torque converter, the lubrication is basically supplied, via the mechanical driving part, which is driven by the internal combustion engine of the motor vehicle while the clutches are basically supplied via the electric driving part. A superimposed oil current control occurs in the valve unit and it is possible to substitute any other driving part for the mechanical driving part. In another development a hydraulic accumulator is used which makes possible the storage of the oil delivered between gear shifts.